Horn of Winter is a death knight ability learned at level 65. It is a group buff that increases attack power. The buff should be kept up at all times, as the attack power gained will increase your damage done.

Patch 3.2.0 (04-Aug-2009): Learning rank 2 of this ability caused players to unlearn rank 1, such that it no longer appears in their spellbook. It is now possible to relearn that rank, and death knights will no longer "forget" rank 1 when they learn rank 2.